OK, fellas, here's a non-Blumo question about timing, but it IS a Mopar...

...we finally got Drake's Duster running today (after about a million dollars worth of parts, or so it seems;))! It runs pretty strong and quite well (except for a slight miss); we had to add a fuel regulator for the Edelbrock 4 barrel, because it was getting about 7 lbs of pressure--we cut it down to 5 and stopped drowning the plugs--and needless to say, I am a happy camper (and so is Drake, as this is his car and I'm just his "free mechanic dad")....
...
BUT....
when I went to set the initial timing with my light and to fine tune it, I can't find the damper mark! 
I've shot it with the timing light, and the darn thing is no where near the cast tab...in fact, it's somewhere under the water pump, and when you turn the distributor to adjust it, it will kill the engine before the mark shows up....otherwise, the car runs fine, but I don't want to risk overheating, bent valves, performance issues, etc., so I want to do this right the first time.
What would cause the mark to vanish? We had it lined up correctly at TDC, with the rotor pointing at the #1 plug, and it started great...but now it's just not there for me to do the initial and total timing. The mark I was using is a groove cut all the way across the damper (there are some other, shorter marks evenly spaced around the circumference also), and the tab is cast with increments and has the big "0" on it.... I know just enough about this stuff to make me dangerous, but this has me confused!
BTW, this is an '86 model 318 with no Lean-Burn, automatic tranny, and a 600 cfm Edelbrock carb, if it matters.
Any ideas?
Rob